The present invention relates generally to the manufacture of semiconductor devices and, in a preferred embodiment thereof, more particularly relates to a two-step lightly doped drain ("LDD") etch process for manufacture of semiconductor devices.
Etching is the removal of material from areas of a workpiece by chemical means, physical means, or a combination of both. Etching in the manufacture of semiconductor devices is typically performed by employing liquids or gases or gas mixtures at particular temperatures and pressures to act upon the workpiece in a manner which subjects the workpiece to conditions achieving a desired removal effect. Special machines, often referred to as etching machines, are often employed in industry to effect such removal by creating conditions therefor and subjecting the workpiece to such conditions. Etching may be employed in the manufacture of a number of devices, including semiconductor devices. A variety of etching processes may be employed in a variety of industries, the foregoing being intended only as a general description of etching and the accompanying results.
Specifically in connection with semiconductor manufacture, etching processes can take place either in a liquid or gas phase. Further, such processes may be either chemical, physical or a combination thereof. Chemical etching in the manufacture of semiconductors removes material from the workpiece by dissolution processes. Physical etching removes such material by bombardment of the workpiece with high-energy ions.
In the typical etch process for manufacture of semiconductor devices, the device to be etched comprises a silicon substrate base having a lattice structure, such base being layered with a gate oxide material. Topping the gate oxide layer are interconnecting strips of a semiconductive material, the poly gate. The poly gate is joined at selected locations on the gate oxide layer atop the silicon substrate base to collectively define the overall semiconductor circuitry of the device. The silicon substrate base/gate oxide layer/poly gate strips assembly is topped with a layer of a spacer oxide which covers both the poly gate and those portions of the gate oxide layer not topped by the interconnecting strips of poly gate.
This device is etched using an etching machine common to the semiconductor device manufacturing industry. Such etch exposes the poly gate, removes the spacer oxide, except for bevelled ridges of spacer oxide which abut the poly gate, and removes from the surface of the silicon substrate base the portions of the gate oxide layer not sandwiched between the poly gate or spacer oxide and silicon substrate base. Such etch process yields a commercially useable form of semiconductor device.
Prior to the present invention, the industry practice has been to perform such an etch for semiconductor manufacture, using an etching machine, under conditions of (1) a gas composition comprising a noble gas, (2) low pressure, and (3) high power. Such an etch causes significant damage to the lattice structure of the silicon substrate base. The damage to the lattice structure is caused by the energetic bombardment of ions in the gas composition, which bombardment breaks the silicon bonds of the silicon substrate base and allows penetration of hydrogen and other atoms interstitially into the silicon lattice. Such an etch also results in damage from polymer formation on the exposed surface of the silicon substrate base. Ion bombardment and radiation due to reactions of the gas composition in the etch chamber, together with heavy metals and other metallic impurities sputtered off during the etch, imparts charge to the silicon substrate base's surface and deposits carbon-containing polymer thereon. Such lattice structure damage and polymer formation results in resistance problems, breakdown of oxides, and adverse hot electron effects in the semiconductor device produced in the etch process. Hereinafter, "damage" is used to refer to both damage to the lattice structure of the silicon substrate base and polymer formation on such base's surface.
It has been determined in accordance with the present invention, that as pressure decreases and power increases in the etch process noble gas ion bombardment, and thus damage to the silicon base therefrom, increases. The industry's perception and practice prior to the present invention has, nevertheless, been that the etch procedure must be accomplished under conditions of high power and low pressure and that a noble gas must be included in the gas composition used in the etch process.
The present invention, an improved LDD oxide spacer etch process, reduces damage caused in an initial etch procedure and further provides for a subsequent removal procedure for etched removal of damage caused in the initial etch procedure. Reducing damage and then removing damage restores the silicon substrate base in the semiconductor device obtained as the final product of the process to essentially that of unetched silicon, reducing the occurrence of contact resistance problems, breakdown of oxides, and adverse hot electron effects in such device.